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Significantly slower than calculator

Old 01-12-21, 04:49 AM
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cubewheels
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Significantly slower than calculator

I've been using this to predict my speed as per power reading by Strava only (I have no power meter, etc...)

Bike Calculator

I ride in a quasi TT posture all the time to train in that position (to adapt to that posture). Strava is giving me average 323 watts and 22 mph speed over a long and relatively flat section of the road with only 0.1% gradient (I've been trying to analyze). Doesn't sound much but I only weigh 125 lbs and I've been training hard in adapting to highly aero positions to help gain some performance in the flats (my weakness as a light rider).

However, despite my very aero position, my equipment is far from aero and efficient. Non aero gravel bike that weighs a whopping 38 lbs!! with 35mm 27 tpi hard "bullet-proof" urban clinchers with butyl tubes (definitely not in the category of low rolling resistance tires), full fenders (I never remove them), Thick quill pedals with no straps, loose fit shorts and shirt, and skateboard helmet.

Is it even possible that Strava is underestimating my power output if it's assuming I'm riding with the best equipment possible at the drops or hoods or something??

Now the calculator is telling me I should be reaching 27.5 mph (with aerobar position which I always do or 25.36 mph in the drops) which is signficantly faster than 22 mph.

Is that discrepancy due to the non-aero equipment and bullet-proof urban tires that I use?
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Old 01-12-21, 10:23 AM
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Yup to that last sentence. Strava power estimates are almost useless except for climbing long steady gradients, steep enough that aero is not a consideration, and then only if one has a wheel sensor. This is my experience after riding with no PM and then repeating with PM. My guess is that you are not producing 5.7 w/kg on the flat. And therefore the calculator is also useless.

The only thing interesting to analyze for training purposes will be your time up various climbs vs. recent workouts. Even that isn't good to obsess over because that road can lead to overtraining. Better to watch morning HRs and HRV and experiment with different training methods and diet. IMO that's a more useful obsession.
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Old 01-12-21, 11:41 AM
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When using that bike calculator, be sure to adjust for time appropriately. The calculator oddly uses hundredths of a minute, rather than 60 seconds per minute. So if our time on a particular segment is, say, 10 minutes and 45 seconds, we'd need to enter 10.75 minutes in the calculator.

The basic formula that calculator uses is pretty much the same that Strava uses, with big generalizations and assumptions about riding position, aerodynamic profile, etc. The main advantage to that online calculator is the ability to enter wind factors. Each of the online calculators I've tried has its own quirks. None of them in entirely intuitive or easy to use, but that particular calculator you linked to is the easiest I've tried, with the exception of the time being in hundredths of a minute, rather than in seconds.

There are other online calculators that use pretty much the same formula, but add the ability to refine approximations based on our riding position, aero profile, etc.

No idea how these compare with a calibrated power meter on a bike. I use those calculators only as references to themselves over a period of time. I have no idea how much power I'm putting out, only how one ride compares with another. I only use the data to compare with other rides on the same routes using the same calculator, with the addition of wind factors if appropriate.
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Old 01-12-21, 10:00 PM
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Originally Posted by Carbonfiberboy View Post
Yup to that last sentence. Strava power estimates are almost useless except for climbing long steady gradients, steep enough that aero is not a consideration, and then only if one has a wheel sensor. This is my experience after riding with no PM and then repeating with PM. My guess is that you are not producing 5.7 w/kg on the flat. And therefore the calculator is also useless.

The only thing interesting to analyze for training purposes will be your time up various climbs vs. recent workouts. Even that isn't good to obsess over because that road can lead to overtraining. Better to watch morning HRs and HRV and experiment with different training methods and diet. IMO that's a more useful obsession.
Yeah, that 5.7 w/kg sounded ridiculous but I do remember breathing hard and really pushing in that segment.

You could be right. I'm comparing the 320 watts average in the flats vs 320 watts average uphill and the uphill version certainly feels harder.
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Old 01-12-21, 10:12 PM
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Originally Posted by canklecat View Post
The basic formula that calculator uses is pretty much the same that Strava uses, with big generalizations and assumptions about riding position, aerodynamic profile, etc. The main advantage to that online calculator is the ability to enter wind factors. Each of the online calculators I've tried has its own quirks. None of them in entirely intuitive or easy to use, but that particular calculator you linked to is the easiest I've tried, with the exception of the time being in hundredths of a minute, rather than in seconds.

There are other online calculators that use pretty much the same formula, but add the ability to refine approximations based on our riding position, aero profile, etc.
Easy is also what I liked about the calculator link I posted and I've tried others before. I think it's extrapolating based on rider weight but I assume it's calculating based on the use of best possible equipment / gear.

I did came across the more comprehensive calculators that also allowed to input rider drag in terms of area. Definitely harder to use. Like getting your frontal area for starters and the fenders would have a drag influence more than the frontal area alone so it gets complicated. Would be nice if there is a simple calculator available that can also extrapolate gear attached like fenders.

The problem is if Strava numbers are not reliable then any calculator would be useless as well.
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Old 01-12-21, 11:15 PM
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Did you enter your bike in as an MTB on Strava? Despite higher RR and potentially higher drag with your flappy clothes, I just don't believe that Strava power is anything but a significant overestimate. With the same system weight but as a taller person with a more upright posture, 22 mph on virtually flat terrain with neutral wind takes me ~250W. Throwing into the mix more mediocre numbers for CdA and Crr would only add about 20W. The only way anyone would need >300W would be if they were bolt upright riding on knobby MTB tires. In fact, I put MTB Cda (0.4) and Crr (0.01) into this calculator to estimate the power necessary to go 22 mph with your weight and average slope and got almost exactly the same estimate that Strava gave you.
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Old 01-13-21, 02:10 AM
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Originally Posted by surak View Post
Did you enter your bike in as an MTB on Strava? Despite higher RR and potentially higher drag with your flappy clothes, I just don't believe that Strava power is anything but a significant overestimate. With the same system weight but as a taller person with a more upright posture, 22 mph on virtually flat terrain with neutral wind takes me ~250W. Throwing into the mix more mediocre numbers for CdA and Crr would only add about 20W. The only way anyone would need >300W would be if they were bolt upright riding on knobby MTB tires. In fact, I put MTB Cda (0.4) and Crr (0.01) into this calculator to estimate the power necessary to go 22 mph with your weight and average slope and got almost exactly the same estimate that Strava gave you.
I entered my bike in Strava as Cyclocross bike weighing 17.3 kilos / 38 lbs with all accessories included in the weight except for water bottle because I don't always bring water bottle.

I have full fenders and rear pannier rack on my bike as well. Saved your link, thanks!

My tires are probably even worse than decent knobby CX tires in terms of rolling resistance. Mine's only got 27 TPI and rubber compound used is quite hard. Definitely not designed for speed but mainly to last long and resist punctures.
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